Countercurrent liquid-vapor fractionation such as used in distillation and adsorption columns is a separational procedure used most frequently in the petroleum and chemical industry. The investment in fractionating equipment, auxiliary operating equipment, piping and operating costs represents a large item in plant costs which it is most desirable to reduce.
In the conventional prior art processing of crude petroleum oil to recover fractions thereof suitable for upgrading in various refinery processing operations, the crude is first distilled or fractionated in an atmospheric distillation tower with residual material separated from the bottom of the atmospheric distillation tower being further separated in a vacuum distillation tower. In this combination operation, gas and gasoline are recovered as overhead products of the atmospheric distillation tower, heavy naphtha, kerosene and light gas oils are taken off as side streams and the residual material is recovered from the bottom of the tower as reduced crude. This residual fraction from the atmospheric distillation tower is then passed to a vacuum distillation tower. The products of vacuum distillation include vacuum gas oil and heavy residual material known as vacuum reduced crude.
In the conventional prior art operation, the crude oil is heated by heat exchange means and furnace means in order to vaporize a portion of the crude to produce the products recovered from the atmospheric distillation tower. The preheated crude normally enters the lower portion of the tower and the vapors therefrom rise through the tower wherein they are cooled in selected stages producing successively lighter liquids which are separately withdrawn as sidestream products. Excess liquid runback known as overflash material joins the liquid portion of the entering crude to form the crude tower bottoms. Steam may be introduced to the bottom of the tower and various side strippers used to remove light material from withdrawn heavier liquid products.
In a large fractionating column where there are multiple withdrawals of the products comprising gas oils and lower boiling materials, the heat supplied must be sufficiently high to cause a more complete vaporization of the crude oil charge for flow upwardly through the tower. The vaporized material is condensed, collected and separated from different levels in the tower. In general therefore, a great amount or more usually an excessive amount of heat is required and provided to vaporize large portions of the crude along with some heavy bottom or residual material in order to provide the necessary amount of reflux material required in the tower above the feed inlet. Without this excess vaporization there is little or no reflux on the tower plates as required for efficient operation.
The present invention is concerned with improving upon an atmospheric-vacuum tower combination operation for separating crude oil which will improve upon the overall economies of the operation and reduce particularly the heat utility requirements of the operation.